A high purity resistant starch lyophilized powder and a method for its preparation

Abstract

This invention belongs to the field of food processing technology, specifically a high-purity resistant starch freeze-dried powder and its preparation method. Using resistant starch rice flour with a dry basis resistant starch content of 30%–35% as raw material, the method employs a dual-enzyme hydrolysis of α-amylase and saccharifying enzyme to specifically remove digestible starch, dextrin, and oligosaccharides from the raw material. After centrifugation and rinsing purification, a resistant starch wet material with a dry basis purity ≥90% is obtained. The finished product is then obtained through pre-freezing, graded freeze-drying, and low-humidity granulation and packaging. High purity and outstanding functionality: This invention utilizes a dual-enzyme synergistic targeted hydrolysis of α-amylase and saccharifying enzyme to specifically remove digestible starch, dextrin, oligosaccharides, and other components from the raw material. Combined with multiple centrifugation and rinsing to remove soluble impurities and ash, the dry basis purity of the resistant starch can be increased from approximately 30% to over 90%, with extremely low impurity content. The physiological efficacy of the product is significantly enhanced, meeting the ingredient standards for high-end functional foods and special medical foods.

Description

Technical Field

[0001] This invention relates to the field of food processing technology, specifically to a high-purity resistant starch freeze-dried powder and its preparation method. Background Technology

[0002] Resistant starch (RS) is a type of starch component that cannot be hydrolyzed and digested by amylase in the human small intestine but can be fermented and utilized by intestinal microorganisms in the large intestine. As a natural functional dietary fiber and prebiotic, it plays an important physiological role in stabilizing postprandial blood sugar, improving intestinal microecology, enhancing satiety, and regulating weight. It is one of the core ingredients in the fields of functional foods and special medical purpose formula foods.

[0003] Currently, the mainstream production processes for resistant starch products are mainly divided into three categories: The first category is the natural extraction method, which directly extracts resistant starch from high amylose corn starch as raw material. The purity of the product can reach 60% to 80%, but the high price of high amylose raw materials, limited production capacity, and high production costs make it difficult to promote and apply on a large scale. The second category is the modification preparation method, which converts ordinary starch into resistant starch through physical modification (wet heat treatment, pressure heat treatment) or enzymatic modification (pullulanase debranching). This process has low raw material costs, but the purity of the resistant starch product is generally only 30% to 50%, with high content of impurities such as digestible starch, dextrin, and ash, significantly weakening its functionality. The third category is the drying and molding process, where existing resistant starch products are generally produced as powdered products using spray drying or hot air drying.

[0004] Existing technologies still have many intractable drawbacks, such as:

[0005] The lack of economically feasible purification technology makes it impossible to increase the purity of resistant starch to over 90% at low cost. The high proportion of impurities limits the physiological efficacy of the product and fails to meet the ingredient requirements of high-end functional foods.

[0006] The processing temperatures of spray drying and hot air drying are both above 100℃. High temperatures can easily cause the gelatinization or degradation of the resistant starch crystal structure, resulting in a decrease in its anti-enzymatic properties and a significant loss of biological activity.

[0007] Ordinary resistant starch powder has a dense particle structure, which makes it prone to clumping together when it comes into contact with water. It disperses slowly and has a poor mixing experience, which limits its application in instant products such as solid beverages and meal replacement powders.

[0008] The production process lacks continuity, with the purification and drying processes being independent and fragmented, making it difficult to achieve continuous and large-scale production of high-purity resistant starch freeze-dried powder. Summary of the Invention

[0009] This invention provides a high-purity resistant starch freeze-dried powder and its preparation method. Using low-cost, low-purity resistant starch rice flour as raw material, the digestible components are removed through dual-enzyme targeted hydrolysis combined with freeze-drying, achieving a resistant starch purity of ≥90%. This simultaneously solves the problems of activity loss during heat drying, poor product solubility, and difficulty in scaling up the process.

[0010] To achieve the above objectives, the present invention provides the following technical solution:

[0011] A method for preparing high-purity resistant starch freeze-dried powder employs a combined process of enzymatic purification and freeze-drying, specifically including the following steps:

[0012] Step (1), raw material preparation: Mix resistant starch rice flour and purified water at a weight ratio of 1:5 to 1:10 to make a slurry, and adjust the pH of the slurry to 5.0 to 6.0; the resistant starch rice flour has a dry basis resistant starch content of 30% to 35% and a fineness of ≥200 mesh;

[0013] Step (2), dual-enzyme synergistic hydrolysis: first, α-amylase is added to the slurry for liquefaction hydrolysis, and then saccharifying enzyme is added after adjusting the pH and temperature of the system for saccharification hydrolysis. The digestible starch, dextrin and oligosaccharides in the raw material are hydrolyzed into soluble sugars, while insoluble resistant starch is retained in the solid phase.

[0014] Step (3), enzyme inactivation and centrifugation: After the enzymatic hydrolysis is completed, the enzyme is inactivated by heating, followed by centrifugation, and the crude solid phase is collected;

[0015] Step (4), rinsing and purification: Add water to the crude solid product, stir and wash, then centrifuge. Repeat the washing 2 to 3 times until no reducing sugar is detected in the washing liquid, and obtain high-purity resistant starch wet material.

[0016] Step (5), Pre-freezing: Spread the resistant starch wet material evenly and then freeze it quickly to make the material completely frozen;

[0017] Step (6), graded freeze drying: the pre-frozen material is subjected to primary drying and secondary drying under vacuum conditions to remove ice crystals and bound water, and a dried cake-shaped material is obtained.

[0018] Step (7), granulation, sieving and packaging: The dried material is crushed and sieved in a low humidity environment and sealed and packaged to obtain high-purity resistant starch freeze-dried powder.

[0019] Furthermore, the specific process of the dual-enzyme synergistic hydrolysis described in step (2) is as follows:

[0020] α-Amylase hydrolysis: Add 0.05% to 0.2% of α-amylase by dry weight to the slurry and incubate at 70 to 80°C for 30 to 60 minutes to liquefy digestible starch;

[0021] Saccharification and enzymatic hydrolysis: Adjust the pH of the slurry to 4.0-5.0, cool it to 55-65℃, add 0.1%-0.3% of saccharifying enzyme by dry weight, and keep it warm for 60-120 minutes to hydrolyze dextrin and oligosaccharides into soluble sugars.

[0022] Furthermore, the preferred parameters for the α-amylase hydrolysis are pH 5.5, temperature 75°C, hydrolysis time 45 min, and the amount of α-amylase added is 0.15% of the dry weight.

[0023] The preferred parameters for the saccharification and enzymatic hydrolysis are pH 4.5, temperature 60℃, hydrolysis time 90 min, and the amount of saccharifying enzyme added is 0.25% of the dry weight.

[0024] Furthermore, the enzyme inactivation conditions in step (3) are as follows: heat the slurry to 95-100℃ and keep it warm for 10-15 minutes; the centrifugation speed is 3000-5000 rpm.

[0025] Furthermore, in step (4), each wash involves adding 2 to 3 times the volume of the solid phase of purified water, and each wash is stirred for 10 to 15 minutes; preferably, the wash is performed 3 times, with each wash adding 2.5 times the volume of the solid phase.

[0026] Furthermore, the pre-freezing conditions in step (5) are as follows: the wet material is laid flat with a thickness of 1 to 2 cm and quickly frozen in an environment of -40℃ to -30℃ for 2 to 4 hours; the preferred pre-freezing temperature is -35℃ and the pre-freezing time is 3 hours.

[0027] Furthermore, the vacuum degree of the freeze-drying system in step (6) is ≤20Pa;

[0028] Primary drying: The temperature of the partition is gradually increased from -20℃ to 0~5℃ and kept at this temperature for 12~24h to allow the ice crystals to sublimate; the preferred heating rate is 1℃ / h and the holding time is 18h. Secondary drying: The temperature of the partition is increased to 30~40℃ and kept at this temperature for 2~4h to remove bound water; the preferred temperature is 35℃ and the holding time is 3h.

[0029] Furthermore, the high-purity resistant starch wet material obtained in step (4) is first subjected to low-temperature vacuum concentration before pre-freezing, and the moisture content of the wet material is reduced to 50% before pre-freezing and freeze-drying.

[0030] Furthermore, in step (7), the granulation and sieving are carried out in an environment with a relative humidity of ≤30%, and the sieving mesh size is 40 to 80 mesh, preferably 60 mesh; the packaging uses aluminum foil bags or light-proof sealed containers, filled with nitrogen or vacuum sealed, and stored at room temperature or refrigerated.

[0031] A high-purity resistant starch freeze-dried powder is prepared by the method described above; the freeze-dried powder has a dry basis resistant starch content of ≥90%, a moisture content of ≤5%, an anti-enzymatic activity retention rate of ≥95%, can be quickly dispersed in water without clumping, and can be stored at room temperature in a sealed container for more than 2 years.

[0032] Compared with the prior art, the beneficial effects of the present invention are:

[0033] (1) High purity and outstanding functionality. This invention uses α-amylase and saccharifying enzyme to synergistically and directionally hydrolyze the raw materials to specifically remove digestible starch, dextrin, oligosaccharides and other components. Combined with multiple centrifugation and rinsing to remove soluble impurities and ash, the dry purity of resistant starch can be increased from about 30% to more than 90%. The impurity content is extremely low, and the physiological efficacy of the product is significantly enhanced, which can meet the ingredient standards of high-end functional foods and special medical foods.

[0034] (2) Low temperature drying, complete preservation of activity. This invention uses freeze drying technology to dehydrate the product. The whole process is in a low temperature and high vacuum environment, which completely avoids the high temperature damage of traditional hot drying process. The crystal structure and molecular conformation of resistant starch are kept intact, and its anti-enzymatic hydrolysis characteristics are retained at a rate of ≥95%, thus preserving the product's biological activity and functional value to the greatest extent.

[0035] (3) Porous structure with excellent instant solubility. During the freeze-drying process, ice crystals sublimate directly, forming a large number of uniformly connected porous structures inside the product. This gives the product particles a large specific surface area. When exposed to water, the water can quickly penetrate into the particles, achieving instant dispersion and dissolution. This completely solves the problems of traditional resistant starch powder clumping and poor dispersibility when exposed to water. It provides an excellent mixing experience and is suitable for various instant food systems.

[0036] (4) The process is green and meets the clean label requirements. The purification process of this invention uses only food-grade enzyme preparations and pure water, without involving any organic solvents or toxic and harmful reagents. There is no risk of chemical residues, the production process is green and environmentally friendly, and the product meets the development trend of clean label and health food.

[0037] (5) Strong stability and long shelf life. This invention controls the product moisture content to below 5% through freeze drying. Combined with sealed oxygen-proof packaging, it can effectively inhibit the growth of microorganisms and starch retrogradation. The product can be stably stored for more than 2 years under normal temperature and sealed conditions. There is no need for cold chain transportation and storage, and the circulation cost is low.

[0038] (6) Strong adaptability and large-scale production: The enzymatic hydrolysis, centrifugation, rinsing and freeze drying processes of this invention can all be implemented using general equipment in the food industry. The process is smoothly connected and can achieve continuous and large-scale production. At the same time, using low-cost, low-purity resistant starch rice flour as raw material, the production cost of high-purity resistant starch is greatly reduced, which has significant economic advantages and industrialization value. Detailed Implementation

[0039] This invention provides the following technical solution: a method for preparing high-purity resistant starch freeze-dried powder, employing a combined process of enzymatic purification and freeze-drying, with the specific steps as follows:

[0040] Step (1) Raw material preparation:

[0041] Resistant starch rice flour was selected as the starting material, requiring a dry basis resistant starch content of 30%–35% and a fineness ≥200 mesh. The raw material can be a commercially available industrial product or can be prepared in-house using existing starch modification processes.

[0042] Preprocessing:

[0043] The resistant starch rice flour is first ultra-finely pulverized by low-temperature airflow at a temperature of ≤40℃ to avoid starch thermal gelatinization. If the raw material contains grain oil, it is degreased at low temperature using food-grade supercritical CO2, and then washed with water to remove free oil.

[0044] The particle surface area is increased by more than 100%, improving the efficiency of the dual enzymatic hydrolysis reaction; the oil barrier layer is removed, resulting in more thorough enzymatic hydrolysis and improved purity and stability of the dry-based resistant starch in the finished product; the product is less prone to oxidation after defatting, extending its shelf life by 6 months.

[0045] The pulverized rice flour is soaked in a 0.05% food-grade sodium bicarbonate weak alkaline solution at a low temperature for 20 minutes (≤30℃) to gently swell the internal pores of the starch granules. It is then neutralized in water before being mixed into a slurry. This method, which requires no high temperature but gently opens the starch granule structure, allows enzymes to fully penetrate and degrade the embedded dextrin, reducing the amount of digestible starch residue and minimizing the number of rinsing cycles.

[0046] Step (2) Mixing the paste:

[0047] Resistant rice flour and purified water are mixed at a weight ratio of 1:5 to 1:10 and stirred thoroughly until no particles clump together, forming a homogeneous slurry. The pH of the slurry is adjusted to 5.0–6.0 using food-grade citric acid or sodium hydroxide to provide a suitable acid-base environment for subsequent α-amylase hydrolysis. Preferably, the flour-to-water weight ratio is 1:8.

[0048] Step (3) Synergistic hydrolysis by two enzymes:

[0049] The method employs stepwise enzymatic hydrolysis with α-amylase and saccharifying enzyme, utilizing the substrate specificity of the enzymes to directionally hydrolyze digestible starch, dextrin, and oligosaccharides in the raw material, converting them into soluble small molecule sugars. At the same time, the structure of insoluble resistant starch is not destroyed, allowing it to remain in the solid phase, thus achieving efficient separation of resistant starch from digestible components.

[0050] The specific process is as follows: ① α-Amylase hydrolysis and liquefaction stage: Add α-amylase to the slurry with the pH adjusted, the amount of which is 0.05% to 0.2% of the dry weight of the raw material; heat the system to 70 to 80°C and keep it at that temperature for 30 to 60 minutes to fully liquefy the linear and branched digestible starches in the raw material and decompose them into short-chain dextrins.

[0051] ② Saccharification and enzymatic hydrolysis, saccharification stage: After the liquefaction reaction is completed, adjust the pH of the slurry to 4.0-5.0 and lower the temperature to 55-65℃ to match the optimal reaction conditions of the saccharifying enzyme; add the saccharifying enzyme to the system at a rate of 0.1%-0.3% of the dry weight of the raw materials; keep the reaction at this temperature for 60-120 minutes to further hydrolyze the dextrin and oligosaccharides produced by liquefaction into soluble small molecule sugars such as glucose and maltose, and completely remove the digestible components.

[0052] As a preferred embodiment, the process parameters for α-amylase hydrolysis are: pH 5.5, reaction temperature 75℃, reaction time 45 min, and the amount of α-amylase added is 0.15% of the dry weight.

[0053] As a preferred embodiment, the process parameters for saccharification and enzymatic hydrolysis are: pH 4.5, reaction temperature 60℃, reaction time 90 min, and the amount of saccharifying enzyme added is 0.25% of the dry weight.

[0054] Step (4) Enzyme inactivation and centrifugation:

[0055] After the enzymatic hydrolysis reaction is completed, the slurry is heated to 95-100℃ and kept at that temperature for 10-15 minutes. The high temperature completely inactivates α-amylase and saccharifying enzyme, terminating the enzymatic hydrolysis reaction and preventing the enzymes from continuing to act and destroy the resistant starch structure in subsequent processes.

[0056] The slurry is then fed into a centrifuge for solid-liquid centrifugation at a speed of 3000-5000 rpm. The solid phase after separation is a crude product enriched with resistant starch, and the liquid phase is an aqueous solution containing soluble sugars, which can be recycled for use in other food processing stages, thereby improving the utilization rate of raw materials.

[0057] Step (5) Rinsing and purification:

[0058] The crude solid obtained from centrifugation is transferred to a washing container, and 2-3 times the volume of the solid is added with purified water. The mixture is stirred and washed thoroughly for 10-15 minutes to fully dissolve any soluble sugars, ash, or other impurities carried in the solid. The mixture is then centrifuged again, and the washing waste liquid is discarded. This washing-centrifugation process is repeated 2-3 times until no reducing sugars are detected in the washing liquid using Fehling's reagent, indicating that purification is complete. The washed solid phase is the high-purity resistant starch wet material, with a moisture content of approximately 60%-70% and a dry basis resistant starch content ≥90%.

[0059] Preferably, the washing is performed 3 times, with each wash adding water at a volume of 2.5 times the solid phase volume.

[0060] Step (6) Pre-freezing:

[0061] The high-purity resistant starch wet material after rinsing and purification is evenly spread in a freeze-drying tray, and the thickness of the material is controlled to be 1-2 cm to ensure the uniformity of freezing and sublimation. The freeze-drying tray is placed in a quick-freezing device and quick-frozen at a temperature of -40℃ to -30℃ for 2-4 hours to completely freeze the free water in the material to form ice crystals, providing a structural basis for subsequent sublimation drying.

[0062] Preferably, the pre-freezing temperature is -35℃ and the pre-freezing time is 3 hours.

[0063] Step (7) Freeze-drying:

[0064] Transfer the fully pre-frozen material, along with the freeze-drying trays, into the drying chamber of the freeze dryer. After sealing the chamber, activate the vacuum system and control the vacuum level inside the chamber to ≤20Pa. Use a staged heating and drying program to sequentially complete the primary and secondary drying processes, preventing the material from collapsing and melting.

[0065] ① Primary drying (sublimation drying): The temperature of the partition is gradually increased from -20℃ to 0~5℃ at a rate of 1℃ / h. The temperature is maintained in this range for 12~24h, so that the ice crystals in the material are directly sublimated and removed under high vacuum, completing the removal of more than 90% of the moisture and forming a porous product structure.

[0066] ② Secondary drying (analytical drying): The temperature of the separator is raised to 30-40℃ and held for 2-4 hours to remove bound water from the material that is tied to starch molecules, further reducing the product's moisture content and improving storage stability. After drying, a cake-shaped dried material with a moisture content ≤5% is obtained.

[0067] Preferably, the primary drying time is 18 hours; the secondary drying temperature is 35°C and the holding time is 3 hours.

[0068] Step (8) Granulation and sieving:

[0069] Turn off the vacuum system, introduce dry and clean air into the drying chamber to restore normal pressure, and remove the cake-shaped dried material. The granulation and sieving process must be carried out in a low humidity environment with a relative humidity of ≤30% to avoid the product absorbing moisture and clumping; use a crusher to lightly crush the dried cake, and then pass it through a 40-80 mesh sieve to obtain a powder product with uniform particle size.

[0070] As a preferred option, the sieve mesh size is 60 mesh.

[0071] Step (9) Packaging:

[0072] The resistant starch freeze-dried powder, after being sieved, is further packaged in a low-humidity environment. Aluminum foil bags or light-proof sealed containers are used as packaging materials, and nitrogen filling or vacuum sealing is performed to isolate oxygen and moisture. The finished product is stored at room temperature or under refrigeration.

[0073] Furthermore, in order to improve production efficiency and shorten the freeze-drying cycle, after rinsing and purifying the wet material in step (5) and before pre-freezing in step (6), a low-temperature vacuum concentration process can be added: under low-temperature negative pressure conditions, some of the free water in the wet material is removed, and the moisture content of the wet material is reduced to about 50%. Then, subsequent pre-freezing and freeze-drying can be carried out, which can significantly reduce the load of sublimation drying and shorten the production cycle.

[0074] This invention also protects the high-purity resistant starch freeze-dried powder prepared by the above preparation method: the product has a dry basis resistant starch content of ≥90%, a moisture content of ≤5%, and an anti-enzymatic activity retention rate of ≥95%; the product has a uniform porous and loose internal structure, and water can quickly penetrate into it after contact with water, and it can be completely dispersed without clumping within 10 seconds, with excellent reconstitution performance; the product has strong chemical and microbiological stability, and can be stably stored for more than 2 years under room temperature and sealed conditions without significant changes in quality.

[0075] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, the embodiments should be regarded as exemplary and non-limiting in all respects. The scope of the present invention is defined by the appended claims rather than the foregoing description, and therefore all changes falling within the meaning and scope of the equivalents of the claims are intended to be included within the present invention.

[0076] Example 1

[0077] The specific steps for preparing high-purity resistant starch freeze-dried powder in this embodiment are as follows:

[0078] Raw material preparation: Take 100g of resistant starch rice flour with a dry basis resistant starch content of 30% and a fineness of 200 mesh.

[0079] Prepare the slurry: Add 800ml of purified water to the rice flour, with a flour-to-water weight ratio of 1:8, and stir thoroughly to form a slurry; adjust the pH of the slurry to 5.5 using food-grade citric acid.

[0080] Double enzymatic hydrolysis treatment:

[0081] Enzyme inactivation and centrifugation: After enzymatic hydrolysis, the slurry is heated to 98°C and kept at that temperature for 12 minutes to completely inactivate the enzyme. Then, the slurry is centrifuged at 4000 rpm for 15 minutes to separate the solid phase and the liquid phase.

[0082] Rinsing and purification: Add 2.5 times the volume of purified water to the solid material, stir and wash for 12 minutes, then centrifuge again; repeat the above washing process 3 times. Finally, Fehling's reagent test showed no reducing sugar in the washing solution, yielding high-purity resistant starch wet material. The dry basis purity of the resistant starch in this wet material was found to be 92.5%, with a moisture content of 68%.

[0083] Pre-freezing: Spread the wet material evenly in the freeze-drying tray, with a thickness of about 1.5cm; place it in a -35℃ environment for quick freezing for 3 hours to completely freeze the material.

[0084] Freeze-drying: Transfer the pre-frozen material into a freeze dryer, evacuate to below 20 Pa, and dry according to the following procedure:

[0085] Granulation and sieving: In an environment with a relative humidity of 25%, the dried cake is lightly crushed and passed through a 60-mesh sieve to obtain a uniform white powder.

[0086] Packaging: Pack the powder into aluminum foil bags, vacuum seal, and store at room temperature.

[0087] Product testing results: The dry basis weight of the product in this embodiment is 22.8g, the dry basis purity of resistant starch is 91.8%, and the moisture content is 3.5%. 1g of powder is added to 20ml of purified water at 25℃ and stirred for 10 seconds to disperse completely without clumping. In vitro enzymatic hydrolysis resistance testing showed that the product retained 96.2% of its enzymatic hydrolysis resistance activity.

[0088] Example 2

[0089] The preparation process of this embodiment is basically the same as that of Example 1, except that: after rinsing and purifying to obtain high-purity resistant starch wet material, it is first subjected to low-temperature vacuum concentration treatment to remove some free water under low-temperature negative pressure conditions, reducing the moisture content of the wet material to 50%, and then proceeding to subsequent pre-freezing and freeze-drying processes.

[0090] Product testing results: In this embodiment, the total freeze-drying time was shortened to 14 hours. The final product had a dry basis purity of resistant starch of 92.0% and a moisture content of 4.0%. The solubility and activity retention were not significantly different from those in Example 1. This embodiment effectively shortened the freeze-drying cycle through a pre-concentration process, further improving production efficiency and making it more suitable for large-scale industrial production.

[0091] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for the preparation of high purity resistant starch lyophilized powder using a combination of bioenzymatic purification and freeze drying, characterized in that, Specifically, the following steps are included: Step (1), raw material preparation: Mix resistant starch rice flour and purified water at a weight ratio of 1:5 to 1:10 to make a slurry, and adjust the pH of the slurry to 5.0 to 6.0; the resistant starch rice flour has a dry basis resistant starch content of 30% to 35% and a fineness of ≥200 mesh; Step (2), dual-enzyme synergistic hydrolysis: first, α-amylase is added to the slurry for liquefaction hydrolysis, and then saccharifying enzyme is added after adjusting the pH and temperature of the system for saccharification hydrolysis. The digestible starch, dextrin and oligosaccharides in the raw material are hydrolyzed into soluble sugars, while insoluble resistant starch is retained in the solid phase. Step (3), enzyme inactivation and centrifugation: After the enzymatic hydrolysis is completed, the enzyme is inactivated by heating, followed by centrifugation, and the crude solid phase is collected; Step (4), rinsing and purification: Add water to the crude solid product, stir and wash, then centrifuge. Repeat the washing 2 to 3 times until no reducing sugar is detected in the washing liquid, and obtain high-purity resistant starch wet material. Step (5), Pre-freezing: Spread the resistant starch wet material evenly and then freeze it quickly to make the material completely frozen; Step (6), graded freeze drying: the pre-frozen material is subjected to primary drying and secondary drying under vacuum conditions to remove ice crystals and bound water, and a dried cake-shaped material is obtained. Step (7), granulation, sieving and packaging: The dried material is crushed and sieved in a low humidity environment and sealed and packaged to obtain high-purity resistant starch freeze-dried powder.

2. The method for preparing high-purity resistant starch freeze-dried powder according to claim 1, characterized in that, The specific process of the dual-enzyme synergistic hydrolysis described in step (2) is as follows: α-Amylase hydrolysis: Add 0.05% to 0.2% of α-amylase by dry weight to the slurry and incubate at 70 to 80°C for 30 to 60 minutes to liquefy digestible starch; Saccharification and enzymatic hydrolysis: Adjust the pH of the slurry to 4.0-5.0, cool it to 55-65℃, add 0.1%-0.3% of saccharifying enzyme by dry weight, and keep it warm for 60-120 minutes to hydrolyze dextrin and oligosaccharides into soluble sugars.

3. The method for preparing high-purity resistant starch freeze-dried powder according to claim 2, characterized in that: The preferred parameters for the α-amylase hydrolysis are pH 5.5, temperature 75℃, hydrolysis time 45 min, and the amount of α-amylase added is 0.15% of the dry weight. The preferred parameters for the saccharification and enzymatic hydrolysis are pH 4.5, temperature 60℃, hydrolysis time 90 min, and the amount of saccharifying enzyme added is 0.25% of the dry weight.

4. The method for preparing high-purity resistant starch freeze-dried powder according to claim 1, characterized in that, The enzyme inactivation conditions in step (3) are as follows: heat the slurry to 95-100℃ and keep it warm for 10-15 minutes; the centrifugation speed is 3000-5000 rpm.

5. The method for preparing high-purity resistant starch freeze-dried powder according to claim 1, characterized in that, In step (4), add 2 to 3 times the volume of the solid phase of purified water for each wash, and stir and wash for 10 to 15 minutes each time.

6. The method for preparing high-purity resistant starch freeze-dried powder according to claim 1, characterized in that, The pre-freezing conditions in step (5) are: the wet material is laid flat with a thickness of 1 to 2 cm and then quickly frozen in an environment of -40℃ to -30℃ for 2 to 4 hours.

7. The method for preparing high-purity resistant starch freeze-dried powder according to claim 1, characterized in that, In step (6), the vacuum degree of the freeze-drying system is ≤20Pa; Primary drying: The temperature of the partition is gradually increased from -20℃ to 0~5℃ and kept at this temperature for 12~24h to allow the ice crystals to sublimate.

8. The method for preparing high-purity resistant starch freeze-dried powder according to claim 1, characterized in that, Before pre-freezing, the high-purity resistant starch wet material obtained in step (4) is first concentrated at low temperature under vacuum to reduce the moisture content of the wet material to 50% before pre-freezing and freeze-drying.

9. The method for preparing high-purity resistant starch freeze-dried powder according to claim 1, characterized in that, In step (7), the granulation and sieving are carried out in an environment with a relative humidity of ≤30% and the sieve mesh size is 40 to 80 mesh; the packaging uses aluminum foil bags or light-proof sealed containers, filled with nitrogen or vacuum sealed, and stored at room temperature or refrigerated.

10. A high-purity resistant starch freeze-dried powder, characterized in that, The freeze-dried powder is prepared by the preparation method according to any one of claims 1 to 9; the dry basis resistant starch content is ≥90%, the moisture content is ≤5%, the anti-enzymatic activity retention rate is ≥95%, and it can be quickly dispersed in water without clumping.